A new method of isometric dynamometry for the craniocervical flexor muscles.

Functionally, motion at the specialized craniocervical (CC) articulations can occur independently of the remainder of the cervical spine, (1) and it is particularly important for fine control of head orientation serving the visual, (2,3) vestibular, and proprioceptive systems. (4-6) Accordingly, the morphology of the CC flexor muscles differs from that of the cervicothoracic flexor muscles. The longus capitis and rectus capitis anterior muscles attach deeply to the front of the cervical spine and superiorly onto the cranium; therefore, these muscles have flexion moments at the CC spine. (7-9) Of these muscles, only the longus capitis can affect cervical motion segments other than the atlanto-occipital joint (cranium-C1) because of its most inferior attachment to the C6 vertebra. (8) In contrast, cervicothoracic flexor muscles either have an extensor moment at the CC spine (sternocleidomastoid muscle) (7) or they attach inferiorly to the cranium so that they are unable to flex the CC junction (longus colli and anterior scalene muscles). (8,9) The only other muscles that are capable of flexing the CC junction are the hyoid muscle group. The hyoid muscle group has extensive attachments originating from the sternum, clavicle, and scapula, with intermediate attachments to the hyoid bone and thyroid cartilage before attaching to the mandible and styloid process. (9) Consequently, these muscles flex all regions of the cervical spine, not selectively the CC region.

Because of their specific function, there has been a trend in research and in clinical practice to evaluate the CC flexor muscles separately from the cervicothoracic flexors. (10-17) When compared with subjects with no history of neck pain, subjects with idiopathic (11,13,16) and traumatic onset (14,17) neck disorders have shown deficits in the contractile capacity of their CC flexor muscles. These deficits include reductions in maximal voluntary isometric contractions (MVIC) (16) and decreases in the capacity to sustain maximal (16) and submaximal (13,14,17) CC flexion contractions. However, there currently are no clinical procedures for measuring the performance of the CC flexor muscles over a range of contraction intensities. We believe that conventional cervical flexion dynamometry methods, (18,19) which resist muscle forces at the forehead, may not isolate contraction of the CC flexor muscles specifically enough to adequately assess their performance.

Watson and Trott (6) described a dynamometry method used with the person being tested in the supine position that specifically assesses CC flexor muscle performance. This method measured the force the CC flexor muscles could exert on a force-sensitive metal bar positioned on the undersurface of the mandible. A pressure sensor, placed under the supporting surface of the head, simultaneously monitored changes in the head pressure on the supporting surface to ensure an isolated CC flexion action. Using this method, Watson and Trott (16) demonstrated intraexaminer reliability (Pearson correlation coefficient [r] =.93) and found reductions in the MVIC of the CC flexor muscles in patients diagnosed with cervicogenic headache compared with subjects with no history of neck pain. As Mayhew and Rothstein (20) pointed out, however, the measurement of muscle force can be problematic because it depends on the distance that the resistance is from the axis of rotation (AOR) of the muscles. Force measurements may vary considerably unless the device is applied at the exact anatomical position for each test, even if muscle tension is identical. Consequently, it may be difficult to compare measurements of force at different points within individuals or at the same point between individuals, and this difficulty compromises the method's potential to be used for dynamic through-range muscle tests in the future.

The purpose of this technical report is to describe a new dynamometry method designed to measure the torque-generating capacity of the CC flexor muscles about the AOR of cranium-C1, with the intent of ascertaining their performance. Muscle forces exert torque to the skeletal system about articular axes. Isometric dynamometry measures torque exerted by muscle groups on the static skeleton in a single plane. In complex multisegmental regions, such as the cervical spine, the torque-generating capacity of planar muscle groups can be simplified by resolving all moments to a single point. (18,19,21) Several researchers have used this method for the cervical flexor muscles by resisting forces at the forehead and resolving moments to the cervicothoracic junction (C7-T1), (18) the C7 vertebra, (19) or the level of the C4 vertebra. (21) These methods refer to isometric tests of cervical flexor muscle performance that, if unrestrained, would produce flexion of the head and cervical spine together on the thorax (cervicothoracic flexors), as is the; case with conventional cervical flexion dynamometry. The method of isometric dynamometry described in this technical report resists forces at the undersurface of the mandible that, if unrestrained, would produce flexion of the head on the cervical spine (CC flexors), while resolving all moments to the 0/C1 motion segment, the principal articulation of CC flexion. We believe that this is the most appropriate method to physically measure the contractile performance of the CC flexor muscles and that it has potential to be used in the future as a clinical measure of these important muscles.

This article is in 2 parts: the first part describes the specifications of the device and calibration procedure, and the second part discusses the reliability of the measurements obtained during the CC flexor muscle performance tests. Muscle performance tests include the measurement of CC flexor muscle MVIC at 3 different points in the CC flexion range and measurement of the steadiness of a sustained isometric contraction (standard deviation of the sustained torque amplitude) at low (20% MVIC) and moderate (50% MVIC) intensities of contraction in the middle neutral position of the CC flexion range. Maximal voluntary isometric contraction of this muscle group has previously been investigated, (16) as have tests of sustained submaximal contractions. (13,14,17) We believe the muscle performance tests, performed as described in this article and using this new technology, may reflect aspects of daily CC flexor muscle function and, therefore, are potential clinical tests of their performance.

Method

Part 1: Dynamometer Specifications and Calibration

The NeckMetrix * dynamometer records 2 simultaneous measurements: isometric CC flexion torque (in newtonmeters) and the dorsal force that the head exerts on the supporting surface (in newtons). The primary feature of the device is an axis and lever arm system to measure the torque-generating capacity of the CC flexor muscle group about the AOR of cranium-C1 (Figure). The AOR for cranium-C1 sagittal-plane motion occurs about the mastoid process, varying from the center of the mastoid process, (22) to the anterior mastoid process, (23) to an area slightly dorsal and cranial to the mastoid process. (24) The concha of the ear, a depression immediately posterior to the bony external acoustic meatus, was chosen as the landmark to which the dynamometer axis was aligned. This landmark approximates the mastoid process, which is otherwise difficult to localize to one point and is blocked from direct vision by the ear.

[ILLUSTRATION OMITTED]

Isometric CC flexion torque measurement. The dynamometer has an adjustable axis permitting alignment to the subject's cranium-C1 AOR landmark. The dynamometer resistance arm consists of 2 metal arms at right angles. One arm is the lever arm that is extended from the axis to a distance so that the adjoining application pad sits under the inferior border of the subject's mandible (Figure, panel B). This resultant lever arm length is adjustable to accomodate different-sized individuals.

A subject's CC flexion effort was resisted at the inferior border of the mandible by the application pad of the dynamometer. The force that the mandible exerted on the application pad was transferred by way of the lever arm, producing torque at the dynamometer axis, which was locked to the load cell deflection arm of the dynamometer. This torque, via the load cell deflection arm, deflected one end of a thin-beam load cell (TBS Series ([dagger])), causing a change in voltage across the load cell. The voltage change was amplified (PM4-SG-240-5E-A ([double dagger])) and transmitted to a personal computer equiped with a custom-written LabVIEW (LabVIEW 6i Virtual Instruments ([section])) program calibrated to convert the amplified voltage to the corresponding torque measurement (in newton-meters). The data were recorded and displayed in real time at a rate of 20 Hz. In addition, the dynamometer axis was adjustable so that it could be freely rotated to the appropriate point in the CC flexion range and then locked to the load cell deflection arm, allowing torque to be measured at the inner, middle, and outer positions of CC flexion range. The LabVIEW software was programmed to zero the torque measurement with the subject completely relaxed immediately prior to applying force to the dynamometer application pad to negate the effects of gravity on the lever arm at different positions relative to the horizontal. The measurement of torque exerted by a subject to the dynamometer axis then could be measured accurately.

Dorsal head force measurement. A secondary feature of the device was a force-sensitive supporting surface for the head included to monitor extraneous motion of the head during the performance of the craniocervical flexor muscle test. Changes in dorsal force on the supporting surface by the head resulting from flexion (a reduction in force) or extension (an increase in force) of the head and neck together in the sagittal plane were measured by a second load cell (ESP Series ([dagger])) attached under the head platform of the dynamometer. The head platform was secured to one end of the load cell, and the other end of the load cell was secured to the bench. The dorsal force that the head exerted on the head platform deflected one end of the load cell, causing a change in the voltage across the load cell that was amplified and converted to the appropriate force (in newtons). In addition, the supporting surface of the head platform was positioned on ball bearings to allow CC flexion effort with minimal frictional effects at the head-platform interface.

Calibration and linearity of the instrument. Amplified voltage outputs of the instrument were recorded for known torque increments at the dynamometer axis, which was achieved by positioning calibrated weights on the horizontal dynamometer resistance arm at staged distances from the axis. Mass ranging from 0.5 to 15 kg was used over lever arm distances of 75 to 135 mm. These parameters adequately covered the range of the lever arm length or force variables observed in pilot trials. The relationship between voltage output recordings and torque increments was modeled by linear regression to determine the least squares regression equation for the voltage data. The LabVIEW program used this equation to convert the amplified voltage to the appropriate torque measurements. The accuracy and linearity of the dynamometer--computer software instrument to measure static torque was then tested by reapplying the weights at various lever arm lengths. The relationship between the recorded torque and the expected torque was modeled by linear regression. The measurement system demonstrated linearity (R>.99) with an offset of -0.072 Nxm.

Part 2: Test-Retest Reliability Study

Subjects. Twenty subjects (15 women, 5 men) participated in the study. Subjects were recruited by printed and electronic advertising within the university. The subjects' mean age was 27.9 years (range = 18-47 years), and they were 12 subjects with neck pain and a control group of 8 subjects with no history of neck pain. Subjects with and without neck pain were included because the intended use of the technology is to compare CC flexor muscle performance between these groups in future studies, and thus the reliability of data obtained with the method in both subject groups warrants investigation. Subjects in the neck pain group were included if they currently had neck pain of greater then 3 months' duration; scored 10 or greater (out of a possible total score of 100) on the Neck Disability Index (average=20.4, range=10-46), indicative of at least mild neck pain and disability (25); and demonstrated signs of cervical spine dysfunction on a physical examination of the neck (abnormal cervical motion, abnormal resistance, and pain provocation to palpation of cervical motion segments). (26) Subjects in the control group were included if they reported no history of neck pain, scored less than 10 on the Neck Disability Index (average=3.5, range=0-8), and had no signs of cervical spine dysfunction on a physical examination of the neck. Volunteers were not considered if they demonstrated neck pain from nonmusculoskeletal causes, neurological signs, any medical disorder that contraindicated physical exercise, or a history of surgery to the cervical spine. After receiving verbal and written information, each subject signed a consent form. This study was granted ethical clearance by the Institute Review Board of The University of Queensland.

Experimental procedure. A test-retest design was used. All 20 subjects were tested on 2 separate occasions spaced 2 weeks apart to minimize training or fatigue effects between sessions. In each session, MVIC recordings were made in the inner, middle, and outer ranges of CC flexion. Sustained submaximal test recordings (20% and 50% of MVIC) were made in the middle range (craniocervical neutral position) only. The MVIC recordings were always completed first, and the order of testing was randomized among subjects but was consistent within subjects and between sessions. The same investigator (SOL) conducted all measurement sessions.

All tests were performed with the subject it: a supine position. To minimize the effects of limb movements on CC flexor muscle performance, the subject's legs were suspended on slings so that the knees and the hips were flexed to 45 degrees and the arms were folded across the chest. Soft straps were attached to the supporting surface and were secured lightly over the subject's shoulders to avoid movement of the trunk on the supporting surface. The subject's AOR landmark (concha of the ear) and the dynamometer axis were aligned with the head in a neutral CC flexion/extension position according to a standard anthropometric neutral position of the head (Frankfort plane). (27) In this craniocervical position, with the subject positioned supine, a vertical line bisects the orbitale and the tragion anatomical landmarks to position the craniocervical spine in a neutral flexion/ extension position. (27) The dynamometer-subject axes were aligned with the aid of a Web camera (QuickCam Pro 4000 ([parallel])) erected perpendicular to the axis of the dynamometer (Figure). Custom-written software (Visual Basic 6.0 (#)) permitted the location of anatomical landmarks on the head (AOR landmark and nostril) and dynamometer reference points (dynamometer axis and lever arm) to be recorded from the Web camera image and replicated on subsequent sessions.

Once the dynamometer lever arm was fitted in a CC spine-neutral position, the location of the anatomical and dynamometer reference points was recorded as the position for isometric torque measurements in the middle CC flexion range. For inner-range measurements, the subject's head and lever arm was positioned in 10 degrees of head flexion from the neutral position. For outer-range measurements, the subject's head and lever arm was positioned in 10 degrees of head extension from the neutral position. The location of the anatomical and lever arm reference points were recorded for each of the 3 positions in the CC flexion range. Ten degrees to either side of the neutral CC position was chosen to represent inner and outer CC flexion ranges, based on reported cranium-C1 extension-to-flexion ranges of motion in the vicinity of 20 degrees (18.63[degrees] [+ or -] 1.51[degrees]). (1)

A visual display unit was then set up in the subject's view. When the subject performed a CC flexion effort against the dynamometer resistance arm, a visual feedback graph was displayed on the screen, increasing or decreasing in accordance with torque production. To avoid bias of performance, the visual feedback graph had no visible units or markings of scale so that, for MVIC tests, subjects were unable to grade the distance the graph moved up the screen visually either between repetitions or between measurement sessions. For sustained submaximal tests (20% and 50% MVIC), visual indicators were displayed on the visual display unit so that the subjects knew how intensely they had to perform a CC flexor contraction to achieve contraction intensities of 20% and 50% of their peak MVIC effort.

All subjects were given standard instructions, familiarization of the testing procedure, and a standard warm-up in all 3 ranges immediately before the trial in that range. Subjects were instructed to nod their head so that their mandible pushed downward on the application pad of the dynamometer to elevate the visual display column maximally. They practiced performing the task, ensuring that the head remained in contact with the head platform and that the teeth remained occluded to minimize the potential contribution of the mandibular depressors.

Warm-up consisted of 4 submaximal repetitions, with each successive repetition at a greater intensity than the previous one, and a fifth repetition to their maximal ability. Three MVIC trials then were performed, with 60 seconds of rest between maximal efforts. Each contraction lasted between 3 and 5 seconds. Subjects were instructed to completely relax between repetitions, ensuring that no active force was placed on the application pad of the dynamometer until commencement of the next trial. The peak of the 3 MVIC trials was recorded as the MVIC score for the range for the session. In addition, the corresponding change in dorsal head force (force at peak torque--force at rest) was recorded. This was done to determine whether the measurements of isometric CC flexor muscle torque and dorsal head force at the moment of peak torque could be recorded consistently in order that the relationship between the 2 measures could be observed and questions about the possible need to control the dorsal head force variable could then be raised and investigated in future studies. This was repeated in all 3 ranges (inner, middle, and outer), with 5 minutes of rest between ranges.

A further 5 minutes of rest was given before subjects performed sustained CC flexor muscle contractions in the middle range position at 20% and 50% of the middle-range MVIC score. First, the contraction at 20% of MVIC was sustained for 65 seconds. Two minutes of rest was given before the subject sustained the 50% of MVIC for 35 seconds. The subject was allowed 5 seconds to reach the requested isometric CC flexor torque amplitude in both sustained tests. Data for this initial 5-second period were discarded; therefore, data from the sustained tests at 20% and 50% of MVIC were analyzed for a 60-second period and a 30-second period, respectively.

For all tests, subjects were blinded to the measurement of dorsal head force. Instead, the focus of the subject was on production of CC flexion torque with visual and standardized verbal encouragement given.

Data management and statistical analysis. For the MVIC tests, means and 95% confidence intervals for both testing sessions (days 1 and 2) were calculated for CC flexor peak torque and corresponding change in dorsal head force in each range for both the neck pain group and the control group. The steadiness of the contraction was measured by computing the standard deviation of the torque amplitude for the 60-second and 30-second contraction periods for the sustained tests of 20% and 50% of MVIC, respectively. Corresponding dorsal head force data were transformed to a change value (in newtons) by calculating the difference between the dorsal head force measurement at rest immediately before commencing the test and the dorsal head force measurement recorded over the first and final 5-second periods of the sustained tests. Means and 95% confidence intervals for both standard deviation and dorsal head force data were calculated for both measurement sessions for the neck pain group, and the control group. Reliability for all measures was expressed for the neck pain and control groups separately by intraclass correlation coefficients (ICC, df= 2,1) and standard error of the measurement (SEM) indexes.

Results

Day 1 and 2 means and 95% confidence intervals for CC flexor MVIC and change in dorsal head force for the neck pain and control groups are displayed in Table 1. Test-retest reliability coefficients for the measurement of MVIC peak torque for the neck pain group (ICC= .87-.93, SEM=0.7-1 Nxm) and the control group (ICC= .79-.92, SEM=0.6-1.4 Nxm) and for the measurement of dorsal head force at peak torque for the neck pain group (ICC=.09-.87, SEM= 18.6-51 N) and the control group (ICC=.49-.96, SEM=8.8-28.4 N) are displayed in Table 2.

Day 1 and 2 means and 95% confidence intervals for CC flexor torque standard deviation and corresponding change in dorsal head force during the sustained tests at 20% and 50% of MVIC for the neck pain and control groups are displayed in Table 1. Test-retest reliability coefficients for these sustained tests for the measurement of standard deviation of torque amplitude for the neck pain group (ICC=.76-.80, SEM=0.01-0.04 Nxm) and the control group (ICC=.07-.74, SEM=0.01-0.13 Nxm) and for the measurement of dorsal head force for the neck pain group (ICC=.85-.91, SEM=2.9-9.4 N) and the control group (ICC=.77-.90, SEM=3.5-8.2 N) are displayed in Table 2.

Discussion

The difference between this new method of dynamometry and conventional cervical flexion dynamometry methods is the measurement of torque around the CC flexion AOR. Previous studies (18,19) have measured torque about the cervicothoracic junction that, when unrestrained, results in cervicothoracic flexion. This new method records torque around the CC junction that, when unrestrained, results in CC flexion (ie, a nodding action of the head on the neck). It needs to be acknowledged that resolving torque measurements to a single axis may be an oversimplified model of the multiarticular and multimuscle CC spine junction. Regardless, our study has shown that the alignment of the dynamometer axis to the AOR of cranium-C1 is a consistent method of measuring isometric CC flexor muscle output and, consequently, that it has potential clinical application.

The results of this study demonstrated that this new method has good reliability in the measurement of MVIC peak torque (ICC=.79-.93) with a relatively small SEM (0.6-1.4 Nxm) and minimal disparities in reliability coefficients calculated for the neck pain and control groups (Tab. 2). These reliability coefficients are similar to those found by Watson and Trott (16) in their measurement of MVIC force of the CC flexor muscles (r=.93); however, unlike our experiment, they ensured that the dorsal head forces remained steady during CC flexor force measurements. Because we wanted to determine the spontaneous dorsal head force response in this preliminary study, dorsal head force was not controlled in our study and subjects were blinded to this measure. A consistent finding across all tests was one of substantial increases in dorsal head force of 154% to 180% during MVIC tests compared with resting dorsal head force and of 38% and 97% during the final 5-second period of the sustained 20% and 50% MVIC tests, respectively. This increase in dorsal head force may be a strategy to improve the neuromuscular efficiency of the CC flexor muscles to gain maximal torque and to sustain torque over time.

In our preliminary investigation, this strategy would appear to be an inconsistent response during MVIC tests and a consistent response during sustained submaximal tests. Change in dorsal head force at peak torque demonstrated large disparities between the reliability coefficients of the neck pain group (ICC=.09-.87) and the control group (ICC=.49-.96) as well as large measurement error (Tab. 2). Very poor consistency of the dorsal head force measurement was noted for the control group in the inner range and for the neck pain group in the outer range. During the sustained 20% and 50% of MVIC tests, dorsal head force measurements at the initial 5-second period (ICC=.80-.90) and the final 5-second period (ICC=.77-.91) both had sound test-retest reliability for both groups. In future studies using this new method to compare CC flexor muscle performance, we will investigate, in large cohorts of subjects with and without neck pain, whether dorsal head force needs to be controlled (as well as the degree of control) to avoid compromising the sensitivity of the CC flexor torque measurement in detecting muscle impairment. We are cautious about adding the control of dorsal head force to the method, because we expect that it will add complexity to the test and may compromise its potential clinical application.

The reliability of data for a measure of contraction steadiness during sustained submaximal CC flexor muscle tests also was assessed by computing the standard deviation of the torque amplitude over the test periods. This measurement may reflect the capacity of the CC flexor muscle group to sustain the torque that may be required in prolonged postural tasks. The measurement showed sound reliability for the 20% of MVIC test (ICC=.74-.80), but large disparities were found between the neck pain group (ICC= .76) and the control group (ICC=.07) for the 50% of MVIC test, with the control group demonstrating very poor reliability coefficients. Again, the implications of not controlling dorsal head force during these tests are unknown at this stage.

We included subjects with and without a history of neck pain in the study because in subsequent studies we will compare muscle performance between these groups and it has been suggested that reliability studies should be performed on population groups of concern. (28) The dynamometer data appeared to be equally reliable for both subject groups, indicating that meaningful comparisons of CC flexor muscle performance can be made in future studies between subjects with neck pain and control subjects. Meaningful group comparisons, however, cannot be made regarding CC flexor muscle performance from this data set because the groups were not matched for age, weight, or sex. This device appears to have application in the evaluation of neck pain and neck muscle impairment.

The technology described in this report is currently a research tool and is not commercially available for clinical application. Subject to device modifications, the technology has the potential to be used for both CC flexion and extension muscle performance, giving it considerable value for future clinical application in assessment and exercise.

Conclusion

A new method of dynamometry for the CC flexor muscles has been described and has shown to yield reliable measurements of MVIC of the CC flexor muscles at 3 points when dorsal head forces are not controlled.

This method appears to have potential clinical application; however, studies that further investigate the validity of measurements of CC flexor muscle performance obtained with this method are needed.

This article was received February 23, 2004, and was accepted November 19, 2004.

References

(1) Worth D. Movements of the head and neck. In: Boyling JD, Palastanga N, eds. Grieve's Modern Manual Therapy: The Vertebral Column. 2nd ed. Edinburgh, United Kingdom: Churchill Livingstone; 1994:53-68.

(2) Andre-Deshays C, Berthoz A, Revel M. Eye-head coupling in humans, I: simultaneous recording of isolated motor units in dorsal neck muscles and horizontal eye movements. Exp Brain Res. 1988;69: 399-406.

(3) Andre-Deshays C, Revel M, Berthoz A. Eye-head coupling in humans, II: phasic components. Exp Brain Res. 1991;84:359-366.

(4) Dutia MB. The muscles and joints of the neck: their specialisation and role in head movement. Prog Neurobiol. 1991;37:165-178.

(5) Keshner EA. Controlling stability of a complex movement system. Phys Ther. 1990;70:844-854.

(6) Winters J, Peles J. Neck muscle activity and 3-D head kinematics during quasi-static and dynamic tracking movements. In: Winters JM, Woo SLY, eds. Multiple Muscle Systems: Biomechanics and Movement Organization. New York, NY: Springer-Verlag; 1990:461-480.

(7) Vasavada AN, Li S, Delp SL. Influence of muscle morphometry and moment arms on the moment-generating capacity of human neck muscles. Spine. 1998;23:412-422.

(8) Kamibayashi LK, Richmond FJ. Morphometry of human neck muscles. Spine. 1998;23:1314-1323.

(9) Moore KL, Dalley AA III. Clinically Oriented Anatomy. 4th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 1999:1012-1028.

(10) Barber A. Upper cervical spine flexor muscles: age related performance in asymptomatic women. Aust J Physiother. 1994;40:167-172.

(11) Falla DL, Jull GA, Hodges PW. Patients with neck pain demonstrate reduced electromyographic activity of the deep cervical flexor muscles during performance of the craniocervical flexion test. Spine. 2004;29: 2108-2114.

(12) Falla D, Jull GA, Dall'Alba P, et al. An electromyographic analysis of the deep cervical flexor muscles in performance of craniocervical flexion. Phys Ther. 2003;83:899-906.

(13) Jull GA, Barrett C, Magee R, Ho P. Further clinical clarification of the muscle dysfunction in cervical headache. Cephalalgia. 1999;19: 179-185.

(14) Jull CA. Deep cervical flexor muscle dysfunction in whiplash. Journal of Musculoskeletal Pain. 2000;8:143-154.

(15) Jull GA, Trott P, Potter H, et al. A randomized controlled trial of exercise and manipulative therapy for cervicogenic headache. Spine. 2002;27:1835-1843.

(16) Watson DH, Trott PH. Cervical headache: an investigation of natural head posture and upper cervical flexor muscle performance. Cephalalgia. 1993;13:272-284.

(17) Jull GA, Kristjansson E, Dall'Alba P. Impairment in the cervical flexors: a comparison of whiplash and insidious onset neck pain patients. Man Ther. 2004;9:89-94.

(18) Jordan A, Mehlsen J, Bulow PM, et al. Maximal isometric strength of the cervical musculature in 100 healthy volunteers. Spine. 1999;24: 1343-1348.

(19) Berg HE, Berggren G, Tesch PA. Dynamic neck strength training effect on pain and function. Arch Phys Med Rehabil. 1994;75:661-665.

(20) Mayhew TP, Rothstein JM. Measurement of muscle performance with instruments. In: Rothstein JM, ed. Measurement in Physical Therapy. New York, NY: Churchill Livingstone Inc; 1985:57-102.

(21) Moroney SP, Schultz AB, Miller JA. Analysis and measurement of neck loads. J Orthop Res. 1988;6:713-720.

(22) White AA III, Panjabi MM. Clinical Biomechanics of the Spine. 2nd ed. Philadelphia, Pa: JB Lippincott Co; 1990:95.

(23) Harms-Ringdahl K, Ekholm J, Schuldt K, et al. Load moments and myoelectric activity when the cervical spine is held in full flexion and extension. Ergonomics. 1986;29:1539-1552.

(24) van Mameren H, Sanches H, Beursgens J, Drukker J. Cervical spine motion in the sagittal plane, II: position of segmental averaged instantaneous centers of rotation--a cineradiographic study. Spine. 1992; 17:467-474.

(25) Vernon H. The Neck Disability Index: patient assessment and outcome monitoring in whiplash. Journal of Musculoskeletal Pain. 1996; 4:95-104.

(26) Jull GA, Bogduk N, Marsland A. The accuracy of manual diagnosis for cervical zygapophysial joint pain syndromes. Med J Aust. 1988; 148: 233-236.

(27) Norton K, Olds T, eds. Anthropometrica: A Textbook of Body Measurement for Sports and Health Courses. Sydney, New South Wales, Australia: University of New South Wales Press; 1996:37.

(28) Rothstein JM. Measurement and clinical practice: theory and application. In: Rothstein JM, ed. Measurement in Physical Therapy. New York, NY: Churchill Livingstone Inc; 1985:1-46.

* UniQuest Pty Ltd, Level 2, Cumbrae-Stewart Bldg, Research Rd, The University of Queensland, Brisbane, Queensland, 4072 Australia.

([dagger]) Transducer Techniques Inc, 42480 Rio Nedo, Temecula, CA 92590.

([double dagger]) Davidson Measurement Pty Ltd, 1-3 Lakewood Blvd, Braeside, Victoria, 3195 Australia. National Instruments Corp, 11500 N Mopac Expwy, Austin, TX 78759.

([parallel]) Logitech Australia Computer Peripherals Ply Ltd, 3-6 The Strand, Dee Why, New South Wales, 2099 Australia.

(#) Microsoft Corp, One Microsoft Way, Redmond, WA 98052-6399.

SP O'Leary, MPhty(Manip), BPhty(Hons), is a doctoral student at The University of Queensland. Address all correspondence to Mr O'Leary at Department of Physiotherapy, The University of Queensland, Brisbane, Queensland, 4072 Australia ([e-mail]).

BT Vicenzino, PhD, MSc, GradDipPhty(Sports), BPhty, is Senior Lecturer, Division of Physiotherapy, The University of Queensland. GA Jull, PhD, MPhty, GradDipAdvManipTher, is Professor and Head, Division of Physiotherapy, The University of Queensland.

All authors provided concept/idea/design and writing. Mr O'Leary provided data collection, and Mr O'Leary and Dr Vicenzino provided data analysis. Dr Jull provided project management, fund procurement, and facilities/equipment.

Table 1.
Group Means and 95% Confidence Intervals (CI) for Day 1
([[bar.X].sub.1]) and Day 2 ([[bar.X].sub.2]) Reliability Sessions
(Inner, Middle, and Outer Craniocervical [CC] Flexion Range) for
Measurement of CC Flexor Muscle Maximal Voluntary Isometric
Contraction (MVIC) Torque (in Newton-meters) and Corresponding Change
in Dorsal Head Force (DHF) (in Newtons): Data Are Reported for Neck
Pain and Control Groups

                        Neck Pain Group (n=12)

                        [[bar.X].sub.1]     [[bar.X].sub.2]
                        (95% CI)            (95% CI)

MVIC
  Inner torque (Nxm)     8.4 (6.8-9.9)       8.4 (6.8-10.1)
  Middle torque (Nxm)   10.6 (8.7-12.5)     11.0 (9.1-12.8)
  Outer torque (Nxm)    10.5 (8.7-12.3)     11.0 (9.2-12.9)
  Inner DHF (N)         71.6 (39.2-104)     84.3 (55.9-113.8)
  Middle DHF (N)        53.0 (25.5-80.4)    76.5 (55.9-98.1)
  Outer DHF (N)         58.8 (16.7-101)     68.6 (42.2-94.1)
20% of MVIC
  SD (a) (Nxm)           0.07 (0.05-0.09)    0.06 (0.05-0.07)
  DHF: first 5 s (N)    15.4 (10.3-20.5)    15.4 (11.1-19.7)
  DHF: final 5 s (N)    17.8 (12.6-23.1)    17.9 (12.1-23.8)
50% of MVIC
  SD (Nxm)               0.15 (0.11-0.19)    0.14 (0.09-0.18)
  DHF: first 5 s (N)    39.5 (26.1-52.9)    45.3 (34.3-56.4)
  DHF: final 5 s (N)    43.8 (28.4-59.3)    49.2 (37.4-61)

                        Control Group (n=8)

                        [[bar.X].sub.1]     [[bar.X].sub.2]
                        (95% CI)            (95% CI)

MVIC
  Inner torque (Nxm)     7.8 (6.3-9.2)       8.5 (7.2-9.7)
  Middle torque (Nxm)    9.7 (7.8-11.7)     10.4 (8.3-12.5)
  Outer torque (Nxm)    10.0 (8.5-11.5)     10.5 (7.9-13.1)
  Inner DHF (N)         70.6 (38.2-104)     70.6 (50-91.2)
  Middle DHF (N)        81.4 (53-109.8)     79.4 (52.9-105.9)
  Outer DHF (N)         68.6 (36.3-102)     74.5 (49-100)
20% of MVIC
  SD (a) (Nxm)           0.06 (0.05-0.07)    0.06 (0.04-0.08)
  DHF: first 5 s (N)    11.5 (5.8-17.2)     10.1 (4.9-15.3)
  DHF: final 5 s (N)    15.5 (9.2-21.7)     15.7 (10.1-21.3)
50% of MVIC
  SD (Nxm)               0.12 (0.08-0.16)    0.19 (0.06-0.32)
  DHF: first 5 s (N)    31.9 (21.2-42.5)    33.7 (21.6-45.8)
  DHF: final 5 s (N)    41.6 (29.3-53.8)    35.7 (20.8-50.6)

(a) SD=standard deviation.

Table 2.
Intraclass Correlation Coefficients (ICC) and Standard Error of the
Measurement (SEM) Values for the Measurements of Craniocervical
(CC) Flexor Muscle Maximal Voluntary Isometric Contraction (MVIC)
(Peak Torque and Corresponding Change in Dorsal Head Force
[DHF]) and Sustained Tests at 20% and 50% of MVIC (Torque
Standard Deviation [SD] and Change in DHF Over Initial and Final
5-Second Periods): Values Are Reported for Neck Pain (n=12) and
Control (n=8) Groups

                   ICC (2,1)             SEM

                   Neck Pain   Control   Neck Pain   Control
                   Group       Group     Group       Group
MVIC
  Inner torque     .93         .89        0.7 Nxm     0.6 Nxm
  Middle torque    .91         .92        1.0 Nxm     0.8 Nxm
  Outer torque     .87         .79        1.0 Nxm     1.4 Nxm
  Inner DHF        .87         .49       18.6 N      28.4 N
  Middle DHF       .79         .96       19.6 N       8.8 N
  Outer DHF        .09         .57       51.0 N      27.5 N
20% of MVIC
  SD               .80         .74        0.01 Nxm    0.01 Nxm
  DHF: first 5 s   .86         .80        3.1 N       3.5 N
  DHF: final 5 s   .91         .77        2.9 N       4.1 N
50% of MVIC
  SD               .76         .07        0.04 Nxm    0.13 Nxm
  DHF: first 5 s   .90         .90        7.0 N       5.1 N
  DHF: final 5 s   .85         .82        9.4 N       8.2 N